Studies on the mammalian microsomal reductive glycosidase show that this enzyme and NADPH cytochrome P450 reductase are the same. The ferric ion complex of adriamycin shows similar action to adriamycin in L1210 cell uptake and inhibition of DNA synthesis. However, the metabolism of the compound is decreased, and the distribution in BALB/c mice shows less uptake by muscle, kidney, and heart. Bleomycin A2 binds to and breaks DNA. We find bleomycin catalyzes the oxidation of Fe2 to Fe ions. We therefore suggest that bleomycin A2 is an enzyme (ferrous oxidase) and may catalyze metal ion oxidation and oxygen reduction in the proximity of DNA leading to DNA damage and cell toxicity.